Which Deworming Program is Right for Your Horse

Rotational Deworming, Strategic Dosing,
Continuous Treatment or Selective Therapy,
Which is the Best?

So Many Parasites, So Little Time

There are over 150 species of internal parasites or worms that can infect your horse. The most common equine parasites to watch for are small strongyles (redworms), ascarids (roundworms), and tapeworms (flatworms), and large strongyles (bloodworms).  

Following is a table that lists each of these parasites, their preferred location inside your horse, and the type of symptoms they cause.

These parasites, left undetected, can wreak havoc on your horse’s internal organs, leading to an ulcerated digestive system, recurrent colic, irritated or damaged lungs, and blood vessel damage. Therefore, it is vital to develop a parasite control program specifically designed for your horse that targets and controls these parasites and worms.

Control Not Eliminate

The goal of any sustainable equine parasite control program, also known as worming or deworming programs, is not to eliminate all parasites but rather to control them.

There are three essential elements to consider when creating a sustainable parasite control program for your horse. Each of these elements plays a critical role in identifying, eliminating, and managing your horse’s internal parasite and worm burden:

• Environment. What goes in your horse will eventually come out of your horse. Keeping your horse’s stall, pasture and dry lot, manure free year-round is critical for guarding against parasitic infection as well as preventing reinfection. Removing feces from stalls and pastures on a regular basis can nearly eliminate the need for deworming treatments.

Deworming Programs - The Beginning

Accompanying the 1960’s introduction of broad‐spectrum equine anthelmintics (dewormers that are effective against multiple types of horse parasites), was the development of treatment programs supposedly designed to totally eliminate your horse’s parasites. However, these programs did not consider the type or number of parasites that may be infecting your horse nor, did they provide any feedback necessary to measure the effectiveness of any treatments.

Research over the past few decades has demonstrated that due to the parasites being a permanent resident in your horse's environment, it is virtually impossible to maintain a parasite‐free horse property, and continued attempts to totally eliminate parasites and worms by repeated use of a chemical dewormer only accelerates parasitic resistance on a property.

Different Strokes for Different Folks

Following are four deworming approaches that have been applied historically in an attempt to control horse parasites:

A major drawback to rotational deworming programs is that instead of testing, these programs recommend rotating deworming products with each treatment at fixed intervals (i.e., every two to three months) all year long. The intent of these programs was to provide proactive coverage against the major types of internal parasites such as blood/redworms, roundworms, and tapeworms that may infect your horse. However, without testing, research now shows that this frequency of treatment is unnecessary, and repeated exposure to excessive toxic chemicals is both wasteful and potentially harmful to your horse.

Additionally, rotating dewormers was supposed to slow parasitic resistance to the products being used. However, the opposite is happening, due to the frequency of use, and some worms have become resistant to certain dewormers.

Because rotational deworming does not include feedback (i.e., testing), most equine parasitologists (these are the folks that study horse parasites) now believe rotational deworming programs are the major factor responsible for the acceleration of parasitic dewormer resistance on horse properties, and therefore should not be considered for long term programs.

2. Strategic Dosing

A strategic dosing program is similar to rotational deworming programs but takes seasonal differences into consideration. Meaning this type of program takes into account when the parasites are most likely to reproduce and therefore shed into a horse's surroundings. In strategic dosing programs, treatments are primarily administered during the active grazing season when parasites are reproducing due to favorable weather. However, these treatments are still performed without testing regardless of the horse's age, health, and surrounding.

Strategic dosing programs do lower the number of treatments, but like rotational deworming, all horses receive the same treatments at fixed times during the year.

3. Continuous, Daily Treatment

A continuous, daily treatment program involves the administration of a dewormer to all horses, either on a year-round daily basis or just daily throughout the grazing season. A continuous program's objective of a daily administration of a dewormer is to kill the ingested immature and recently emerged adult parasites before they invade and damage your horse's internal organs.

Though horses receiving daily treatments should maintain a low egg count while receiving the treatment, recent investigations have shown that even with daily dosing, parasite egg counts are not reduced to zero, and seasonal or gradual increases are still observed. Repeated use of the same daily deworming will also accelerate parasitic resistance to that product.

4. Selective Therapy

As compared to the three other deworming programs, selective therapy reduces the number of treatments and thereby slows parasitic resistance on a horse property by considering the type and number of parasites infecting a horse.

A recent National Animal Health Monitoring Systems for equines has shown that 80% of the parasites on a property come from up to 30% of the horses (see Table 2).

Table 2- Suggested guidelines for classifying horses into different levels of strongyle egg shedding and the expected percentage of the horse population belonging to each group (Kaplan and Nielsen, 2010).